What is the make/model of your drive?
I too have had this exact problem, with my Samsung SM-352B 52x24x52x16 Combo drive, even with the newest firmware.
When I use old 8x rated TDK cdrs, my drive allows me to burn at 32x (crazy!). When I use new DataWrite 40x cdrs, I only get 24x speed, or less, showing up as selectable. This happens with any CD burning software I try (nero, roxio, cd mate, etc) and is due to the drive detecting the disc, and deciding that the media is not compatable.
When no cdr is inserted in my drive, I am able to select any speed all the way up to 52x, but as soon as I insert a 40x DataWrite (blue dye) cdr, I only see 24x, 16x, or 8x speeds as selectable.
According to Samsung, the cause is either, incompatable media, or Buffer Underrun Prevention (JustSpeed for Samsung) setting a lower speed after examining my system. I have already eliminated JustSpeed as the cause, thus leaving media compatability as the culprit. Sure enough, after testing (using CD Speed), I discovered that my Samsung drive does not like blue coloured media, and even has difficulty reading these discs.
You need to sample a variety of media to find one that your drive likes. Unfortunately, I still have a couple of hundred blue cdrs to use up first.